A common requirement in optical systems is the ability to control or stabilize the polarization state of light in an optical fiber. An example of this application would be the field of coherent optical communication systems, in which a local oscillator laser is mixed with the incoming signal light. The resulting difference signal carrying the information is at a maximum when the local oscillator and signal lasers are aligned in polarization at the receiver, and complete loss of signal is possible if these two lasers are significantly misaligned.
Conventionally, the solution to controlling the polarization state of light travelling in optical fiber is through the use of a polarization controller containing a number of polarization active elements which transform the polarization state in specific ways depending on the applied force acting on each element. For example, a lithium niobate based controller possesses a number of stages the birefringence of which may be influenced through the application of an electric field. An alternative known scheme exploits the sensitivity of high birefringence fiber to an external pressure which is generated from a piezoelectric element.
Common amongst most of these schemes is the difficulty in obtaining a precise and deterministic polarization transformation. For instance, lithium niobate suffers from bias drift with temperature, while high birefringence fiber has a very strong sensitivity to both temperature and pressure. Even when these variables are compensated for, the polarization controller itself will have fiber tails in order to integrate it with the other components in the system, and these tails themselves will have an unknown and variable effect on the polarization state of light entering and exiting the device.
For these reasons conventional polarization control relies on a feedback technique, whereby some parameter correlating to the desired output state is used in a control loop to iteratively adjust the polarization controller for the desired transmitted output state. An example of a strong feedback parameter device would be a polarizer. The output of the polarizer is maximized (or minimized) through iterative and continuous adjustment of the controlling elements of the polarization controller. A disadvantage of this technique is the lack of agility with which the system can respond to changes in the input state of polarization. Since the polarization transfer function of the fiber tails is unknown, every change in input polarization state requires an iterative adjustment to the controller elements to rediscover the best configuration of the elements within the polarization controller. This can be slow.
It is known that the state of polarization at the input of a conventional polarization controller of the type described hereinabove can vary arbitrarily and rapidly. Since conventional polarization control can be slow this is not well adapted to such a rapidly changing situation.